In water, ground and down in the bedrock, there are a heat source free of cost. Use of heat from sea, surface ground heat and bedrock heat is a secure, safe and environmental heating technique. The heat energy can be transmitted to an existing, conventional waterborne heating system and also used for hot-water production.
In a geothermal heating system, the hear is extracted from a bore hole, a so called drilled energy well. The collector is the pipe, which comprises a heat transfer medium in the form of a so called heat transfer liquid, that convey heat through the heated heat transfer medium and that also convey the cooled heat transfer medium back in a closed cycle.
In a surface ground heat plant, a several hundred meter long tubing is buried to a frostproof depth and arranged in coils. The collector tubing collects the heat that occur in the ground and use it to vaporize the cooling agent of the heat pump.
Sea heat works fundamentally in the same way as surface ground heat. The energy that exists stored in the sea water and in bottom layer is utilized. The collector tubing is put on the bottom of a water course.
There are various kinds of heat pump collectors. The kind of collector, that is the most frequent today, is called U-pipe collector. According to that principle, a separate, closed pipe is arranged in a drilled hole. This is for example carried out such that a continuous elongated pipe of plastics, suitably polyethylene, is bent on the middle such that it forms a U-shape. The lowest part of the “U” of the pipe is arranged on the drilled hole, and subsequently the pipe is advanced down into the drilled hole. Thus, the U-pipe forms a single continuous line for the heat transfer liquid in a closed cycle, in direction from the heat pump down into to drilled hole and back out of the drilled hole and further back to the heat pump in the one and the same pipe.
For installations of geothermal heating there are also a so called three-pipe collector, which is a variant of a collector that comprises a pipe for conveyance of the heat transfer liquid down into the drilled hole, which pipe has a branching-off to two pipes that transports the heat transfer liquid back from the drilled hole and further to heat pump. The term “single pipe collector” in this disclosure is a common term for the coils of collectors and collector tubing mentioned above that are intended for sea heat and surface ground heat. In the single pipe collector, the heat transfer liquid can be lead in a pipe with essentially the same cross-sectional area along the whole longitudinal extension of the collector.
Another type of collector according to the prior art is the so called coaxial collector. An inner pipe is arranged in an outer pipe. The pipes are welded together to one unit that subsequently is installed in a drilled hole. The purpose with this technique, having “a pipe in a pipe” is that it is desirable to avoid mixing for too long time of the cooled heat transfer liquid, that is brought down into the bore hole, with the heated heat transfer liquid, that shall be carried up in the collector. By providing an outer pipe with large diameter, a slower flow in the outer pipe having large diameter is achieved, whereby a positive output is reached when it comes to heat absorption quality. The coaxial collector is not found in any considerable extent on the market today, since this technique involves large installation and production costs. However, the coaxial collector is generally considered to be more effective than the conventional single pipe collector.
DE 20 2004 007 567 U1 discloses a coaxial collector according to the well known kind, described above. This comprises a connecting pipe for inflow of heat transfer liquid to an outer pipe, that comprises an uneven surface structure on the inside of the outer pipe. The heated heat transfer liquid is returned via an inner pipe up through a connecting pipe.
US 2007/0023163 A1 discloses a coaxial collector according to the well known kind as described above. This comprises a connecting pipe for inflow of heat transfer liquid in an inner pipe, that ends just above the bottom of an outer pipe, arranged outside the inner pipe. The heat transfer liquid is heated during circulation in the space between the outer pipe and the inner pipe, while guided via a helical structure (“turbulence generator” or turbulence generating structure”) arranged on the outside of the inner pipe and further conveyed to a heat exchanger via a pump.
The wall thickness on the collector pipe is optimized such that the heat transfer medium obtain a maximal absorption of heat, the pipes becomes easy to handle and such that the return weight strive straight down into the bore hole, in the case with a drilled energy well. The length is adapted as desired.
However, a problem with the traditional single pipe collector is that the absorption of energy from the surrounding water in the energy well is not optimal. There is a large need for a more effective utilization of the energy. This also applies to the single pipe collectors that are intended for sea systems and surface ground heat plants.